1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a fixing belt to apply heat to the recording medium bearing the toner image. FIG. 1 is a sectional view of a fixing device 50R1 including a fixing belt 25. As illustrated in FIG. 1, the fixing belt 25 is looped over a heating roller 22 (inside which a heater 21 is provided) and a fixing roller 23, in a state in which a tension roller 24 biased by a spring applies tension to the fixing belt 25. A pressing roller 26 is disposed opposite the fixing roller 23 via the fixing belt 25 and presses the fixing belt 25 against the fixing roller 23 to form a fixing nip between the pressing roller 26 and the fixing belt 25. As a recording medium P bearing a toner image passes through the fixing nip, the fixing belt 25 heated by the heater 21 via the heating roller 22 and the pressing roller 26 apply heat and pressure to the recording medium P to fix the toner image on the recording medium P.
In order to provide a shortened warm-up time and reduced power consumption compared to the fixing device 50R1 using the fixing belt 25, the fixing device may instead employ a fixing film formed into a loop and having a smaller heat capacity than the fixing belt 25. FIG. 2 is a sectional view of a fixing device 50R2 including a fixing film 14. As illustrated in FIG. 2, a ceramic heater 13 supported by a stay 12 via a holder 11 is provided inside the loop formed by the fixing film 14. A pressing roller 16 presses the fixing film 14 against the ceramic heater 13 to form a fixing nip between the pressing roller 16 and the fixing film 14. As a recording medium bearing a toner image passes through the fixing nip, the ceramic heater 13 applies heat to the recording medium via the fixing film 14 and simultaneously the pressing roller 16 applies pressure to the recording medium. Thus, the heat and the pressure fix the toner image on the recording medium.
With the fixing film 14 having a smaller heat capacity than the fixing belt 25, the fixing device 50R2 can be heated to a predetermined fixing temperature by turning on the ceramic heater 13 immediately before an image forming operation is started. Thus, the fixing device 50R2 is warmed up quickly while at the same time reducing power consumption.
However, the fixing device 50R2 using the fixing film 14 has a drawback in that the fixing film 14 may wear due to friction generated between the stationary ceramic heater 13 and the rotating fixing film 14 that slides over the ceramic heater 13. Accordingly, the worn fixing film 14 with a roughened inner circumferential surface may increase friction between the fixing film 14 and the ceramic heater 13, resulting in unstable rotation of the fixing film 14 and increased driving torque of the fixing device 50R2. The fixing device 50R2 also has a drawback in that the fixing film 14 is heated by the ceramic heater 13 only at the fixing nip, and therefore the fixing film 14 is coolest when entering the fixing nip. Accordingly, when the fixing film 14 is rotated at a high speed, the fixing film 14 may not have been heated to a desired fixing temperature by the time the fixing film 14 enters the fixing nip and contacts the recording medium there.
To address these problems, the fixing device may instead employ a combination of a looped fixing belt and a cylindrical metal heat conductor instead of the fixing film 14 and the ceramic heater 13. For example, the metal heat conductor fixedly mounted inside the looped fixing belt receives heat from a heater provided inside the cylindrical metal heat conductor, and then transmits the heat to the fixing belt. A pressing roller presses the fixing belt against the metal heat conductor to form a fixing nip between the pressing roller and the fixing belt through which the recording medium bearing the toner image passes. With this configuration, the entire fixing belt is heated by the metal heat conductor, which faces substantially the entire portion of the fixing belt as the rotating fixing belt slides over the metal heat conductor. Further, the inner diameter of the fixing belt is set greater than the outer diameter of the metal heat conductor to reduce friction between the metal heat conductor and the fixing belt.
However, the fixing belt and the metal heat conductor have a drawback in that the fixing belt may slacken at a position downstream from the fixing nip in the direction of rotation of the fixing belt. Specifically, the rotating pressing roller pulls the upstream portion of the fixing belt provided upstream from the fixing nip in the direction of rotation of the fixing belt to the fixing nip in such a manner that the upstream portion of the fixing belt is stretched over the metal heat conductor. By contrast, the rotating pressing roller does not stretch the downstream portion of the fixing belt provided downstream from the fixing nip in the direction of rotation of the fixing belt, slackening the downstream portion of the fixing belt. Accordingly, the slackened fixing belt heated by the metal heat conductor contacts the recording medium bearing the fixed toner image at the position downstream from the fixing nip in the direction of rotation of the fixing belt for a longer time, and therefore heats the recording medium excessively, causing hot offset of the toner image fixed on the recording medium. Moreover, the slackened fixing belt disturbs separation of the recording medium discharged from the fixing nip from the fixing belt, causing jamming of the recording medium.